How to set up keyboard shortcut for bounding box viewport shading?
Go over to the 3D View Header, right click over the Viewport Shading Mode menu and choose the option Online Python reference.

That should take you to the Blender's Official Python API Documentation where you can learn all about it.

There you will learn that the the viewport_shadeproperty is an enum that accepts [‘BOUNDBOX’, ‘WIREFRAME’, ‘SOLID’, ‘TEXTURED’, ‘MATERIAL’, ‘RENDERED’]string values.

The one you are looking for would hence be BOUNDBOX.